Amplitude limiting device for amplifying systems



June 21,. 1932.

N.- BISHOP AMPLITUDE LIMITING- DEVICE FOR AMPLIFYING SYSTEMS I FiledFeb. 7, 1930 01 /C/ E 51,001 P 7/ Ourjva/ 5 L Loao ditto; wags PatentedJune 21, 1932 NATHANIEL BISHOP, OF BRIDGEIORT, CONNECTICUT AMPLITUDELIMITING DEVICE FOR AMPLIFYIN G SYSTEMS Application filed February 7,1930. Serial No. 426,578.

\ This invention relates to amplitude limiting devices for amplifyingsystems, and hasfor an object to produce a device whereby the output ofany amplifying system may be limited to a predetermined amount so thatany undesired interference which produces -a greater voltage than thedesired output nected a transmission networ voltage may be limited inamplitude to the same or slightly reater value than the desired output.T us, for example, when used in a radio receiving system it preventssurges, static, transients in near-by power circuits and similar sourcesof interference from producing an effect in the output of the amplifiercircuit greater than the peaks of the desired signal.

In general, the theory of operation is as follows: Across the out utterminals of the amplifier in the amplif ying s stem is conhaving thecharacteristic that if an alternating voltage is applied across itsinput there will be no output from the network until the voltage isincreased to a predetermined value. Beyond this redetermined or criticalvalue a voltage will e present across the output of the network whichwill have a wave shape similar to that portion of each half cycle of theamplifier output voltage which is above this critical value for thenetwork. If, then, this output voltage from the network is introducedinto the input of the amplifier a hundred and eighty degrees out ofphase with the input voltage to the amplifier, the effect will be toprevent the peaks of the input wave from exceeding a value predeterminedby the setting of the constants of the network. In a like manner theoutput of the amplifying system is limited so as not to exceed apredetermined value.

In the accompanying drawing I have shown two arrangements of devices forsecuring this operation and effect. In this drawing Fig. l is a diagramof an amplifying system with a limiting network employing vacuum tubesapplied thereto, and

Fi 2 is a similar diagram showing a limiting network employing a neontube or similar discharge device.

grid bias potential and R is a potentiometer where current will How inthe plate circuit Referring to the first system illustrated in Fig. 1,this method makes use of two three element vacuum tubes V and Vconnected in a push-pull arrangement. B is a source of allowingadjustment of the negative grid bias applied to the grids of the tubs Vand V A is a source of filament potential for these tubes, and B is asource of plate potential for these tubes. The network includes theinput transformer T the primary of which is connected across the output10 of the amplifier 11, the input impedance of the transformerbeing'high in comparison with the load impedance, the load beingrepresented by the element 12, such for example as a loud speaker; Inthe plate circuit of the vacuum tubes V and V is connected an outputtransformer T and the secondary of this output transformer T isconnected to the secondary of the input transformer T of the amplifier11 so that the voltage across the secondary of the transformer T due tothe output of the limiting device, will be a hundred and eighty degreesout of phase with the input voltage to the amplifier.

In the operation of this device, the potentiometer R is adjusted so thatthe peaks of the desired wave fall just below the point of tubes V and VIn other words, the bias is made far enough below' cutofi' so that thestrongest desired signal will produce a voltage on the grids of tubes Vand V ust below the value which will allow plate current to flow. Then,if any interference which produces a greater voltage than the desiredsignal is introduced into the amplifier, the cutofi voltage in thenetwork will be passed and the network will produce a voltage across theinput of the amplifier a hundred and eighty degrees out of phase withand similar to each half of the cycle of the input wave which exceedsthe cutoff voltage, thereby limiting the output of the amplifier due tothis interference to a value only slightly greater than the desiredsignal. By adjustment of the grid bias as by varying the setting ofpotentiometer B, it is possible to make the device operative at anyoutput level of the amplifier which may be desired.

Referring now to Fig. 2 there is illustrated in this figure a method ofsecuring this effect which employs a limiting network making use of aneon tube or similar discharge device having the characteristic that itwill pass current in either direction only after the applied voltage hasreached the critical value or break-down voltage. In this arrangement,the primary of the step-up input transformer T of the network isconnected across the output 10 of the am )lifier 11 the same as in thearrangement of ig. 1. This transformer is designed so that the impedancelooking into its circuit is high in comparison to the impedance of theload 12. Across the secondary of this transformer T is connected thepotentiometer R which affords means for adjusting the voltage applied tothe circuit containing the discharge device N and the secondary of theinput transformer T of the amplifier 11. When current flows through thedischarge device N, it passes through the secondary of the inputtransformer T in such a way as to produce a voltage across the inputtothe amplifier 11 one hundred and eighty degrees out of phase with theinput voltage to the amplifier.

The adjustment of this device is similar to that of Fig. 1 in that thepotentiometer R is set so that the desired signal just fails to ignitethe discharge device N or is set just below the point where distortionis noted on the peaks of the desired signal. Then, if the system issubjected to interference which produces greater voltages across theoutput of the amplifier 11 than the desired signal, the voltage producedby the peaks of this interference will ignite or break down thedischarge device N and it will pass current impulses the wave shape ofwhich will be similar to each half cycle of the output wave whichexceeds the break down voltage for which. the discharge tube circuit isad usted. This current is introduced into the input of the amplifier 11through the secondary of the transformer T one hundred and eightydegrees out of phase with the corresponding peaks of the input voltageto the amplifier, thereby limiting the possible peak voltage across theoutput of the amplifier, to a value only slightly in excess of thedesired signa Both of these devices may be used as a safety valve on anyamplifying system to preventsurges, static, transients from nearby powercircuits and similar sources of inter erence from producing an effect inthe output circuit of the amplifier greater than the peaks of thedesired signal. As an example, the device has been applied to the poweramplifier of several types of radio receivers with success. It has beendemonstrated that while attempting to hear a weak station through stronglocal interference from a spark coal and without this device in thecircuit, it was impossible to hear the station at all. With this devicein the circuit, it was found possible to understand what theannouncerwas saying. Such interference as intermittent crackling, etc., of muchgreater volume than the program giving the effect of a series ofdeafening crashes when this device was not used, was reduced by the useof this device to a dull clicking.

The second method, employing the discharge tube or device N, is limitedin its application to systems where sufficient voltage is available tocause the tube to ignite at the desired voltage level, but due to itsextreme simplicity, it is ideally suited for application on modern radioreceivers using power amplifiers. \Vith this device it is possible tolimit the output of an audio frequency amplifier to a. predeterminedvalue, and it is especially adapted to be used in conjunction with radioreceivers subjected to intermittent interference which would normally belouder than the signal or program being received. A neon lamp has acharacteristic such that its internal impedance is very high at audiofrequencies if the voltage applied to the lamp is lower than the valuenecessary to light it (or the break-down voltage). As soon as thiscritical or break-down voltage is reached its impedance becomes lowenough to pass current during that portion ofeach one-half cycle of theimpressed voltage that the voltage is equal to or greater than thecritical voltage;

' No claim is made that these devices will in any way affect backgroundinterference or noise of lower volume than the desired signal. Theywill, however, make a marked difference in theeffect of high levelinterfer-- ence'and in such a way as to have no detrimental effect'onthe quality of the desired signal.

Having thus set forth the nature of my invention, what I claim is:

1. In an amplifying system the combination with an amplifier of atransmission network having its input connected across the output ofsaid amplifier and having an impedance sufficiently great to preventcurrent flow through said network until a voltage is impressed thereongreater than the voltage of the desired output from the amplifier andmeans connecting the output from said net work across the input to theamplifier to impress the output thereon 180 out of phase with the inputto the amplifier.

2. In an amplifying system the combination with an amplifier of atransmission net work having the characteristic that if an alternatingvoltage is applied across its input there will be no output from saidnetwork until the voltage exceeds a predetermined value, meansconnecting the input of said network across the output of the amplifier,and means connecting the output from said network across the input tothe amplifier to impress the output thereon 180 degrees out of phasewith the input to the amplifier.

3. In an amplifying system the combination with an amplifier of atransmission network having the characteristic that there will be nooutput therefrom until the voltage across its input is equal to apredetermined value, means connecting the input of said network acrossthe output of the amplifier, and means connecting the output from saidnetwork across the input of the amplifier to impress the output thereon180 out of phase with the input to the amplifier.

4. In an amplifying system the combination with an amplifier of atransmission network having the characteristic that there will be nooutput therefrom until the voltage across its input is equal to apredetermined critical value, means for varying the impedance of the network to var said critical value, means connecting the input of saidnetwork across the output of the amplifier, and means connecting; theoutput of said network across the input of the amplifier to impress theoutput thereon 180 out of phase with the input to the amplifier.

5. In an amplifying system the combination with an amplifier of atransmission network including an input transformer having its primarconnected across the output from the ampli er, means in said network topre- .vent output therefrom until the voltage on the input to thenetwork reaches a predetermined amount, and means connecting the out utof the. network to the input to the ampli er to deliver the output fromthe network' to the input to the amplifier 180 out of phase with theamplifier input.

6. In an amplifying system the combination with an amplifier of atransmission network including an input transformer having its primaryconnected across the output from the amplifier, means in said network toprevent flow of current therethrough when the voltage on the input tothe network is less than a predetermined amount, means in the networkwhereby the said predetermined limit may be varied, and means fordelivering the output from the network to the input to the amplifier onehundred and eighty degrees out of phase with the amplifier input.

7 In an amplifying system the combination with an amphfier of atransmission network including an input transformer having its primaryconnected across the output from the amplifier, a circuit including thesecondary of said transformer connected to the input to the amplifier todeliver the output from said network to the. input to the ampli fier180degrees out of phase with the amplifier input, and a neon tube insaid circuit to prevent current flow to the amplifier input when thevoltage in the circuit is.less than a predetermined amount,

input to the amplifier to deliver the output,

from said circuit to the input to the amplifier 180 degrees out of phasewith the amplifier input, and a neon tube in said circuit to preventoutput therefrom when the voltage; in

the amplifier output is below a predeter mined limit.

9. In an amplifying system the combination with an amplifier of atransmission net:--- 'work having its input connected across the outputfrom the amplifier and its output con.- nected to the input to theamplifier to deliver the output from said net work to the input to theamplifier 180 degrees out of phasewith the amplifier input, and a neontube in.

said network to prevent current flow there through when the voltage onthe input to the network is less than a predetermined amount.

10. In an amplifying system the combination with an amplifier of atransmission network having its input connected across the output fromthe amplifier and its output connected to the input to the amplifier todeliver the output from said net work to the input to the amplifier 180degrees out of phase with the amplifier-input, a discharge device insaid network to prevent current flow therethrough when the voltage onthe input to the network is less than a predetermined amount, and meansto cause the discharge device to pass current at different inputvoltages.

In testimony whereof I afiix my signature.

NATHANIEL BISHOP.

